The present invention relates generally to valves, and more particularly to mechanisms for counteracting or controlling forces imparted on such valves due to the flow of fluid.
In fluid flow designs, particularly high pressure applications, high pressure fluid flow through valves creates forces which tend to close the valve, particularly when the valve is close to the cracking point or otherwise in or near to a xe2x80x98fully closed positionxe2x80x99. The flow force is often a significant contributor to the force required to stroke a valve. In hydromechanical applications, where the valve is stroked from a mechanical source with ample force, this is not generally a problem. The difficulty arises when a valve is stroked from an electromagnetic device, such as a solenoid or electric actuator which has distinct force limitations. Such force limitations in turn limit the largest size possible for single-stage electrohydraulic devices. To achieve larger valve sizes, a two-stage system or some technique for reduction of the flow forces must be implemented. The invention described herein, refers to a novel method for reduction of flow forces.
Of particular interest in this invention are the xe2x80x98flow induced forcesxe2x80x99 tending to close the valve, although flow forces may in some configurations be opposite in direction. In high pressure applications, the main reason for these flow induced closing forces is because the crack or opening at one axial end of the valve causes a localized pressure decrease as fluid escapes. However, the pressure at the other axial end of the valve does not experience the same localized decrease in fluid pressure and continues to remain high such that the opposing axial forces at the axial ends of the valve are unbalanced tending to urge the valve closed. Closing forces also occur due to fluid acceleration and angle of exit of the flow. This applies but is not limited to poppet valves, spool valves, or other similar linearly translatable valves. These forces are sometimes referred to as xe2x80x9cflow induced forcesxe2x80x9d or xe2x80x9chydraulic reaction forcesxe2x80x9d.
The problem with xe2x80x98flow induced forcesxe2x80x99 is that the forces needed to stroke a valve can be very high. The conventional prior art approach of dealing with xe2x80x98flow induced forcesxe2x80x99 has been to either use a two-stage system or incorporate large actuator forces and return spring forces into a valve assembly for the purpose of overcoming the xe2x80x98flow induced forcesxe2x80x99. According to one common valve assembly arrangement, a large spring force is used to overcome the xe2x80x98flow induced forcesxe2x80x99 to move the valve to an open position while a solenoid is used to overcome the spring force to close the valve. These xe2x80x98flow induced forcesxe2x80x99 diminish in a substantial exponential manner as the valve continues to open resulting in an unduly large spring force when the valve is fully open. The solenoid must overcome this large spring force to close the valve from the fully open position.
The large force actuators require more electrical power both in terms of currents and voltages. A typical voltage available in engine environments is often 24V. However to drive the larger solenoids and achieve acceptable slew times, voltages in the region of 100-120V are sometimes necessary. For a 24V voltage supply this will normally require a step-up power supply. Thus, this is an undesirable drawback with current systems.
Still further, a moving valve may open fully and then experience an unwarranted reclosure due to the kinetic energy of movement in combination a particular spring and flow forces. This is a problem that exists in certain applications that can be difficult to solve.
Still further, for a given electric actuator force and for the case of poppet valves in high pressure applications, a hold-in force is required to keep the valve at its seat to prevent leakage. In high pressure applications, this force can be significant. For the case of an electric actuator and return spring arrangement, the net force available for sealing depends on the value of the spring preload. Again this requires higher actuator and return spring forces.
In summary, the typical prior art approach of simply incorporating a lot of xe2x80x9cmusclexe2x80x9d by including large spring forces and large solenoid forces is undesirable for a number of reasons, that typically include any of the following: cost and size drawbacks associated with providing larger solenoids (or other actuators) and larger spring forces; assembly difficulties; larger stress and/or impact loads; higher driver voltage requirements and/or a decrease in valve speed. There have been some attempts to reduce these xe2x80x98flow induced forcesxe2x80x99 or otherwise control other forces in valves. However, prior approaches have not sufficiently solved the problems in the art, cause other difficulties/problems or are not specific to translatable valve.
It is a primary object of the present invention to provide a better solution to the problem of xe2x80x98flow induced forcesxe2x80x99 in valve assemblies and/or otherwise provide an improved mechanism for controlling fluid forces in a predetermined manner.
In this regard, it is a more specific objective of the present invention to provide a valve assembly that controls xe2x80x98flow induced forcesxe2x80x99 better to achieve one or more of the following advantages depending upon the application, including but not limited to: smaller actuator forces, smaller spring forces, reduced operating currents and voltages of electric actuator driven systems, reduced part stress, reduced impact loads, increased speed or responsiveness, lower actuator cost, improved reliability, improved valve assembly lifespan, and/or reduced risk of valve reclosure.
It is a further objective to provide a mechanism for reducing xe2x80x98flow induced forcesxe2x80x99 that is relatively easy to implement across a number of different types and sizes of valves, including poppet valves, spool valves, and other reciprocating valves.
A further objective is to provide a counterbalance to the xe2x80x98flow induced forcesxe2x80x99, which varies according to the position of the valve. One such fluid mechanism would deliver a force which is similar in shape to the xe2x80x98normalxe2x80x99 closing (negative) forces but opposite in direction. This implies a mechanism which can sharply increase the positive counteracting axial forces as the valve approaches its closed position or vice-versa in the opposite direction of movement. One goal may be to achieve or approach the state of a fully xe2x80x98balanced valvexe2x80x99 throughout its stroke range or another may be to shape the flow force curve to a given profile.
A still further objective of the design is to use the momentum of the exiting fluid to impart a force on the valve, by turning the fluid through an angle as it impacts the valve. By dissipating available energy to effect a force on the valve, the pressure loss caused by introducing any additional restrictions can be minimized. The angle of deflection of the fluid can be controlled by the angle of the impact surface in the intermediate region or by its shape. Generally turning the fluid through a 90 degree bend will impart the maximum momentum onto the valve.
In accordance with these and other objectives the present invention is directed toward a mechanism for fluidically controlling xe2x80x98flow induced forcesxe2x80x99 in a valve assembly. The valve assembly comprises a valve body defining first and second flow passages. A valve is movable in the valve body along an axis to control fluid flow between the first and the second flow passages. The valve has first and second control edges. The second control edge is spaced axially and radially from the first control edge such that the valve includes a radially extending surface between the first and second control edges. First and second restrictions are formed between the first and second control edges and the valve body, respectively. The first and second restrictions form a intermediate pressure region such that when the first and second flow passages are at different fluid pressures with fluid flowing past the valve, the intermediate region acts on the radially extending surface of the valve. The pressure of the intermediate region can be used to counteract opposing axial xe2x80x98flow induced forcesxe2x80x99 acting on the valve or otherwise provide a desired valve bias.
The present invention is also directed to a method of counteracting xe2x80x98flow induced forcesxe2x80x99 in a valve assembly comprising controlling and restricting the fluid flow from a pressurized high pressure region radially outward to a low pressure region with a first restriction between the valve and the valve body. The restricted fluid flow is then again restricted with a second restriction between the valve and the valve body. This forms an intermediate pressure region between the first and second restrictions having a fluid pressure between that of the low and high pressure regions. The intermediate pressure region applies a counteracting axial force to the valve using the fluid pressure of the intermediate pressure region acting on the valve. The counteracting axial force provided by this design counteracts the otherwise naturally occurring xe2x80x98flow induced forcesxe2x80x99.
The present invention solves many of the various problems discussed in the background section. For example, by significantly reducing the xe2x80x98flow induced forcesxe2x80x99, the xe2x80x98loadxe2x80x99 on the solenoid is reduced and fast slew times can be achieved with larger electrical actuators operating at 24V supply and utilizing relatively low values of spring force. Also, by approaching a balanced valve hydraulically, the variations for different flow rates and pressures are reduced and thus there is less cycle to cycle variation of the closure point of the valve in fuel injection applications. A further advantage is that by shaping the flow forces at fully open to be balanced or even positive (tending to open the valve), undesirable side effects of valve reclosure such as delayed spill or secondary injections in a fuel injection system can be prevented. For fuel injection and other applications, the invention can reduce the return spring force such that more force is available for sealing or alternatively the same size actuator can seal a bigger diameter valve. In sum, the invention has wide appeal across many different applications and resolves many of the problems that have been heretofore experienced.
Other objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.